In a manufacturing field such as a factory, a communication system (a field network) is configured by connecting field devices such as a PLC (Programmable Logic Controller), an inverter, and a thermometer, and various processes are performed while performing data communications between the field devices. In such a communication system, token passing using a LAN (Local Area Network) specification such as Ethernet® techniques is employed, for example, so as to guarantee the realtime property of data communications performed between the field devices. In recent years, because many field devices are connected to a communication system, it has been desired that data handled by the field devices is efficiently processed in a short time.
For example, in a data link method described in Patent Document 1, two master stations are provided in a data link system. The master stations perform data communications with local stations under their control.    Patent Document 1: Japanese Patent Application Laid-open No. H5-252168